1. Field of the Invention
This invention relates to a structure of a heterostructure bipolar transistor (hereinafter referred to as "HBT") in which a germanium layer is formed on a gallium arsenide body and to a method for manufacturing the same by using a molecular beam epitaxial growth.
2. Description of the Related Arts
The HBT uses an emitter region composed of a semiconductor material which has a broader energy band-gap than the semiconductor material constituting a base region. One example is described as an NPN bipolar transistor in "Journal of Applied Physics", Vol. 59, No. 2, pp. 495 to 498. Epitaxial layers of P-type germanium 21 and N-type germanium 22 are grown on an N-type gallium arsenide substrate 20 (FIG. 1(a)) and the N- and P-type germanium epitaxial layers 21 and 22 are partially mesa-etched (FIG. 1(b)). Then, a mask 70 for defining an emitter region is formed and boron ions are implanted (FIG. 1(c)). Finally, a collector electrode 23, a base electrode 24 and an emitter electrode 25 are selectively formed by a lift-off method of evaporated metal. An HBT can thus be fabricated (FIG. 1(d)).
The region of gallium arsenide substrate 20 into which boron is implanted becomes a region of gallium arsenide 26 having high resistance. Therefore, it becomes possible to inject electrons into P-type germanium layer 21 and to extract them from the N-type germanium layer 22, so as to make the transistor operate as an NPN bipolar transistor using gallium arsenide substrate 20 as the emitter, germanium layer 21 as the base and germanium layer 22 as the collector. Here, the N-type germanium layer 22 and the P-type germanium layer 21 can be formed by molecular beam epitaxial growth method.
However, arsenic in the gallium arsenide substrate 20 diffuses into the P-type germanium layer 21 in the heterojunction of gallium arsenide and germanium. Thus, the P-type germanium layer 21 is doped to an N-type with an impurity concentration of as high as at least 10.sup.18 cm.sup.-3. In order to dope the germanium epitaxial film 21 directly grown on the gallium arsenide substrate 20 to the P-type, it is, therefore, necessary to highly dope an acceptor above 10.sup.18 cm.sup.-3 so as to compensate the impurity doped from the substrate 20. Accordingly, it is difficult to grow a germanium layer directly on a gallium arsenide substrate to have a P-type of such a high impurity concentration. A base layer in a bipolar transistor is very thin. Therefore, the formation of such a base layer with high controllability is very difficult technically. Moreover, since electron and positive hole mobilities decrease in the semiconductor epitaxial film whose impurity is thus compensated, base resistance of the base layer as well as the electron travelling time in the base layer increase and, thus, they exert adverse influences on the electrical characteristics of the transistor.
This problem occurs not only in HBT, in particular, but also in other applications using the germanium epitaxial film directly grown on gallium arsenide body such as in a P-channel field effect transistor using the germanium epitaxial film on gallium arsenide substrate.
As above-mentioned, HBT utilizing the heterojunction between gallium arsenide body and germanium layer epitaxially grown on the gallium arsenide body involves the problem that arsenic in the gallium arsenide body diffuses into the grown germanium layer at the interface therebetween so that the germanium layer is doped with the N-type impurities. To accomplish NPN type HBT, the germainum layer which is directly grown on gallium arsenide body as a base layer must be a P-type epitaxial film having an extremely small film thickness. Under such condition where diffusion of arsenic occurs as described above, a P-type dopant having a higher concentration must be added to the extremely thin germanium layer. This is not only difficult technically but also invites the drop of mobility of the positive hole or electron in such a base layer whose impurity is compensated for, so that the electrical characteristics of the transistor become deteriorated.
The problems described above occur not only in HBT but also in other applications using the germanium epitaxial film on gallium arsenide in general.